Future firefighters have their work cut out for them. Perhaps nowhere does this hit home harder than in Australia, where in early 2009 a persistent drought, high winds, and record high temperatures set the stage for the worst wildfire in the country’s history. On February 9th, now known as “Black Saturday,” the mercury in Melbourne topped 115 degrees Fahrenheit (46.4 degrees Celsius) as fires burned over 1 million acres in the state of Victoria—destroying more than 2,000 homes and killing more than 170 people, tens of thousands of cattle and sheep, and 1 million native animals.

Even as more people move into fire-prone wildlands around the world, the intense droughts and higher temperatures that come with global warming are likely to make fires more frequent and severe in many areas. (See table of regional observations and predictions.) For southeastern Australia, home to much of the country’s population, climate change could triple the number of extreme fire risk days by 2050.

Although fires typically make the news only when they grow large and put lives or property at risk, on any given day thousands of wildfires burn worldwide. Fire is a natural and important process in many ecosystems, clearing the land and recycling organic matter into the soil. Some 40 percent of the earth’s land is covered with fire-prone vegetation. A number of plants—such as giant Sequoia trees and certain prairie grasses—need fire to propagate or to create the right conditions for them to flourish.

Fire patterns have changed over time as human populations have grown and altered landscapes by clearing forests, allowing pasture animals to overgraze grasslands, and importing new plant species. Across parts of the western United States, for example, cheatgrass, an invasive annual adapted to frequent burns, has supplanted native brush, desert shrub, and perennial grasses that typically experience longer intervals between fires. In other areas, mixed-age and mixed-species forests have been replaced by single-species plantations where flames can jump easily from tree to tree. The result, instead of a low-intensity restorative fire, is a fire so hot that it can cause lasting harm to soils.

Humans have also altered fire patterns through deliberate suppression. After 1910, when a severe wildfire charred more than 3 million acres of western U.S. forest in just two days, the strong desire to protect timber resources gave life to a policy of quickly extinguishing fires. For decades firefighters proved remarkably successful in this endeavor, but the upshot was that forests became so loaded with fuel that a blaze that evaded control could quickly grow into a dangerous megafire.

Now policies are shifting in many places to let some fires proceed naturally or through preventative controlled burns; yet by warming the planet, we may be relinquishing even more control than we bargained for. Higher average global temperatures mean extremes are in store: even as climate change brings more flooding in some areas, other places will be plagued by droughts and extended heat waves. As the temperature rose between the 1970s and early 2000s, for instance, the share of total global land area experiencing very dry conditions doubled from less than 15 percent to close to 30 percent. A hotter, drier world burns more readily. Global warming could be pushing us into a new regime of larger, longer-burning, more intense fires as well as fires in places that historically have been hard to ignite, like moist tropical forests.

Already there is evidence of the connection between higher temperatures and wildfire. Anthony Westerling of Scripps Institution and colleagues found a marked uptick in forest fires in the western United States since the mid-1980s, with the wildfire season lengthening by 78 days over the last 15 years compared with the preceding 15 years. The fire season length and the duration of each fire rose in concert with regional spring and summer temperatures, which were an average 0.87 degrees Celsius higher in the later period. Higher temperatures are melting mountain snow cover earlier in the spring, leaving less moisture for the summer and giving fires a better chance to spread. And while human land use certainly has had a direct effect on wildfire patterns throughout the West, the biggest increase in U.S. wildfire frequency has actually occurred in the largely untouched mid-elevation Northern Rockies forests, implicating climate change.

Farther north, Alaska’s and Canada’s boreal forests recently have experienced more-frequent fires, releasing enough carbon to transform them in some years from net absorbers to net emitters. Between the 1960s and the 1990s, the total area burned more than doubled. Higher temperatures have extended the range of the tree-damaging spruce budworm into new territory and allowed spruce beetles, no longer delayed by cold winters, to complete their typical two-year life cycle in just a single year. Drought has limited the efficacy of the trees’ defenses. Together the insects and the drought are leaving millions of acres of dead wood in their wake, providing fuel for wildfires. Overall, a warmer climate is predicted to double the area burned in northern Canada by 2100; in Alaska, the area could double by as early as 2050.

In other parts of the world fire regimes are changing and are projected to change even more as the planet heats up. Over much of Europe fire frequency decreased during most of the twentieth century, and expanding forests soaked up carbon. Now, however, some areas may be starting to see more fires. Between 2000 and 2006, some 50,000 fires burned each year in the Mediterranean region, compared with 30,000 a year in the 1980s, though the total area burned did not increase, in part because of more vigilant firefighting.

During Europe’s record 2003 heat wave, which killed over 50,000 people, an estimated 650,000 hectares of forest burned continent-wide. Although the number of fires during this warm and dry year was not particularly high, the area burned was a record. More than 5 percent of Portugal's forest area burned, four times the 1980–2004 annual average, resulting in economic damages exceeding 1 billion euros. If future warming is not kept in check, hot and dry summers like 2003 could happen as frequently as every other year, dramatically increasing wildfire risk.

For Southeast Asia, the extreme 1997–98 El Niño brought a major drought to the region, allowing some of the most severe fires in recent history to burn in Indonesia, the Philippines, and Laos. Fires set to clear land jumped from grasslands and shrublands to logged forests and peat swamps, where they burned underground. For months Southeast Asian skies were hazy from smoke. Nearly 10 million hectares burned in Indonesia alone, affecting 23 of 27 provinces and costing more than $9 billion.

During that same El Niño, more than 20 million hectares burned in Latin America, wreaking damages of up to $15 billion. In 2001 the following El Niño brought more drought and put a frightening one third of Amazon forests at risk of burning. With a temperature rise of more than 3 degrees Celsius—well within the range projected for this century barring rapid and dramatic action to curb carbon emissions—much of South America is likely to see more frequent wildfires.

Just as a weakened immune system leaves a person vulnerable to otherwise innocuous germs, the combination of logging, road construction, and intentional burning to clear forests for cattle ranches, farms, and plantations has fragmented the world’s tropical forests, increasing their vulnerability to fire. Piling higher temperatures on top of such stresses could completely undermine forests’ resilience. For the massive Amazon rainforest, we risk reaching a tipping point where recurrent droughts dry out the landscape enough so that small fires can turn into devastating conflagrations.

We all rely on trees to soak up greenhouse gases and store carbon. If large swaths of forest go up in flames, it could set into motion a vicious cycle, where more wildfires in turn release more carbon into the atmosphere. Stabilizing climate, and doing so quickly, takes on a new urgency when it means averting an inferno on earth.

Janet Larsen is the Director of Research for the Earth Policy Institute.